This invention relates to broadband free space optical communications systems.
At this time, there exists a substantial, and rapidly growing demand for multi-megabit communications to homes or businesses. This high band width is required in support of computer access, web surfing, multi-media, voice, E-mail, and video applications. Unfortunately, the common broadband access network transport technologies of DSL (Digital Subscriber Lines including Asymmetric, Symmetric, and Rate Adaptive), cable modems, wireless broadband, satellite, and fiber to the subscriber, all suffer from problems limiting their deployment DSL re-uses the copper in the ground, and therefore, is fast and inexpensive to install, but it suffers from technology limitations related to loop characteristics, and is limited to a few Mbps. Regulatory and spectral allocation issues limit wireless broadband in networks, although they offer fast and inexpensive deployment. Fiber and satellite deployment have plenty of bandwidth, but are prohibitively expensive. The users are expecting the same quality, reliability and speed of access, currently enjoyed in large corporate Intranet installations, to be ubiquitously available everywhere.
Consequently, a point-to-point terrestrial microwave system is sometimes used. In this service, an antenna is mounted on a high tower, having line of sight access to a subscriber who requires the service. One such antenna is provided for each such subscriber. The antennas of the subscriber and the tower based antennas are aligned, so that each is aimed at the other. The system is expensive. One tower mounted antenna is required for each subscriber, and it is necessary to align that antenna with the subscriber""s location when the system is installed. Further, the reliability of the system is limited to the reliability of each antenna, and its"" associated electronics.
Another system that is used is LMDS, (Local Multi-Point Distribution System). In this system, a central omni-directional antenna serves a plurality of subscribers, each of which have a directional antenna aimed at the central antenna. The disadvantage of this arrangement is that all the subscribers must share a single slice of the radio spectrum; the radio spectrum must be individually licensed because the radio signal is broadly radiated.
The above problem is solved, and an advance is made over the teachings of the prior art in accordance with this invention, wherein an optical, line-of-sight transceiver (OLST), mounted on a suitably high platform is used to transmit a steerable modulated LASER beam to each of a plurality of customers within the sight of that OLST; when an OLST is steered to a particular customer, the OLST also receives signals from that customer to establish the other direction of communication, and to provide feedback as to whether the transmitted steered signal is being properly received. Advantageously, such an arrangement sharply reduces the number of OLSTs required to be mounted at tower sites, and eliminates the installation problem of manually aiming each tower mounted OLST that is required in a point-to-point distribution system. An OLST can serve customers in an angular region, called a sector of the tower. A few dozen to a few hundred subscribers share a single virtual pipe, (one sector of the tower), with several hundred Mbps bandwidth available to be shared by the customers of a sector.
In accordance with one preferred embodiment a group of several solid angle, (azimuth and elevation), sector OLSTs are mounted on a tower to serve the different sectors surrounding a tower. In Applicants"" preferred embodiment, a group of eight such OLSTs serving eight different sectors is mounted as part of each hub. Advantageously, this limits the angle over which each OLST needs to be steered.
In accordance with Applicants"" preferred embodiment, each customer is provided with a slice of time out of an overall repetition cycle. During this time slice, information is exchanged between the sector OLST and the customer using one wavelength downstream and a different, widely spaced wavelength upstream. In Applicants"" preferred embodiment, the repetition cycle is typically 20 milliseconds. The slice of time allocated depends on how much data is to be transferred between the hub, and a particular customer. The amount of data which can be exchanged during each overall time cycle is limited by the demands of other customers being served during the same time cycle, and the electronic, mechanical and optical design of the system elements. Advantageously, the cycle limits the latency of information transmitted in the two directions to an amount that supports interactive services and voice transmission.
In accordance with Applicants"" preferred embodiment, a pair of optical galvanometers, or other beam steering apparatus are used to control the angle of a transmitting and receiving mirror so that the LASERs are aimed at, and received from a single subscriber with which a particular OLST is currently communicating. Alternatively, other beam steering apparatus such as crystal-based beam deflectors can be used for steering the beam. Optical galvonometers are manufactured by, for example, General Scanning Corporation and Cambridge Scientific Corporation. Using this technology, the LASER beam and the receiving apparatus can be moved rapidly from one customer to another, thus, minimizing the overhead for moving the LASER beam. The scanning system moves the beam to the next customer when one of two events happen; either all of the data to be transmitted in each of the two directions has already been transmitted, or the allocated time slice for the customer has been used up. The size of the time slice is chosen to ensure that the beam visits each active subscriber at least once within the maximum interval needed to provide an acceptable latency, such as 20 milliseconds, to minimize the delay of critical services, such as voice. The data to drive the LASER, and the data received by the optical assembly, are sent/to, received/from a switching arrangement, for example, a router, and thence, to/from the broadband backbone network interconnecting the towers with each other, and with long distance facilities.
In accordance with Applicants"" preferred embodiment, an overall control unit directs local control units within each OLST to determine which subscribers are to be controlled from which OLSTs. Advantageously, this permits subscribers in sector overlap areas, which can be served by either of two sectors, to be served by the less busy sector, thus increasing the overall capacity of the system.
Each subscriber""s terminal consists of two fixed optical assemblies, both of which are carefully aligned with a central hub at installation, and permanently fixed. The first optical assembly is a LASER that is modulated by the upstream data in a burst, carefully timed to coincide with the downstream burst that results when the hub""s scanner points at the subscriber. The other optical assembly is a lens and an optical receiver that terminates the downstream link. Data received from the downstream receiver and transmitted to the hub, passes over a standard local area network, and interconnects the subscriber terminal and the various pieces of equipment the user needs to connect to a backbone network for interconnecting hubs and other transmission and switching systems.
In the preferred embodiment, different wavelengths are used for the downstream and upstream signals; advantageously, this minimizes upstream/downstream crosstalk.